A current implantable cardioverter-defibrillator (ICD) detects tachyarrhythmia when ventricular rate exceeds a specified tachyarrhythmia rate threshold, and classifies the detection as a sudden onset of tachyarrhythmia if exceeding the specified tachyarrhythmia rate threshold occurs together with a sudden change in ventricular rate.
The performance of such an algorithm can be less than certain. For example, the current method of sudden tachyarrhythmia onset determination may not be appropriate when the ventricular rate increases quickly, but then only gradually increases as it crosses the tachyarrhythmia rate threshold. Such a scenario would not be deemed a sudden onset of tachyarrhythmia by such an algorithm.
Another potentially inappropriate operation of such an algorithm occurs when a sudden onset tachyarrhythmia is detected, and anti-tachyarrhythmia therapy is delivered, but where the delivered anti-tachyarrhythmia therapy fails to convert the ventricular heart rhythm from a tachyarrhythmia to a normal heart rhythm and instead results in slowing the ventricular rate below the tachyarrhythmia rate threshold, followed by a gradual increase back above the tachyarrhythmia detection threshold.
In yet another situation, the ventricular rate quickly exceeds the tachyarrhythmia rate detection threshold in a manner indicative of sudden tachyarrhythmia onset, but no anti-tachyarrhythmia therapy is delivered before the ventricular rate spontaneously briefly slows slightly below the tachyarrhythmia rate threshold (thereby resetting the sudden onset status), and then gradually increases back above the tachyarrhythmia rate threshold. Because the current ICD device resets the sudden onset status when the ventricular rate slows below the tachyarrhythmia rate threshold, the subsequent ventricular rate increase above the tachyarrhythmia rate threshold would not be deemed a sudden onset.
In general, inappropriate determination of sudden tachyarrhythmia onset can result in an inappropriate therapy delivery response, including failure to deliver therapy when needed.